7 6 



HUMAN BIOLOGY 



must have many of i\\Q yellow elastic fibers of connective 

 tissue in their walls. They are lined with an exceedingly 

 thin membrane of- epithelial cells through which oxygen 

 and carbon dioxid are exchanged. In the walls of the air 

 cells there is a network of capillaries (Fig. 71). The dark 

 red blood comes into these capillaries from the pulmonary 

 arteries, and is changed to a bright red by the time it 

 leaves them to enter the pulmonary veins. The air leaves 

 the lungs warmer, moister, and containing more carbon 

 dioxid than when it entered. 



Most of the mucous membrane lining the air passages 

 has a surface layer of ciliated cells. Cilia are tiny thread- 

 like projections (Fig. 72) which con- 

 tinually wave to and fro, the quicker 

 stroke always being outward ; for their 

 function is to remove particles of dust 

 and germs that may find entrance to 

 the air passages. When the mucus 

 containing the dust is raised nearly to 

 the larynx, it may be thrown out by 

 coughing. Near the opening of the nos- 

 trils are placed many hairs, hundreds 

 of times larger than cilia, through which the air is strained 

 as it enters the nose. Hairs are multicellular ; cilia are 

 parts of cells. See Animal Biology, Fig. 14. 



The Lungs.- -The entire chest cavity is occupied by the 

 lungs except the space occupied by the heart, the larger 

 blood vessels, and the gullet. The right lung has three 

 lobes, or divisions, and the left lung has two lobes. The 

 lungs are light pink in early life, but become grayish and 

 darker as age advances. This change is more marked in 

 those who dwell in cities, or wherever the atmosphere is 

 smoky and dusty. The lungs are covered and inclosed by 



FIG. 72. CILIATED 



CELLS, lining the air 



passages. 



